Improving the long-term stability of perovskite solar cells is critical to the deployment of this technology. Despite the great emphasis laid on stability-related investigations, publications lack consistency in experimental procedures and parameters reported. It is therefore challenging to reproduce and compare results and thereby develop a deep understanding of degradation mechanisms. Here, we report a consensus between researchers in the field on procedures for testing perovskite solar cell stability, which are based on the International Summit on Organic Photovoltaic Stability (ISOS) protocols. We propose additional procedures to account for properties specific to PSCs such as ion redistribution under electric fields, reversible degradation and to distinguish ambient-induced degradation from other stress factors. These protocols are not intended as a replacement of the existing qualification standards, but rather they aim to unify the stability assessment and to understand failure modes. Finally, we identify key procedural information which we suggest reporting in publications to improve reproducibility and enable large data set analysis.
The
operational stability of perovskite solar cells (PSCs) remains a limiting
factor in their commercial implementation. We studied the long-term
outdoor stability of ITO/SnO2/Cs0.05((CH3NH3)0.15(CH(NH2)2)0.85)0.95PbI2.55Br0.45/spiro-OMeTAD/Au cells, as well as the dynamics of their degradation,
under simulated sunlight indoors and their recovery in the dark. The
extent of overall degradation was found to depend on processes occurring
both under illumination and in the dark, i.e., during the daytime
and nighttime, with the dynamics varying with cell aging. Full recovery
of efficiency in the dark was observed for cells at early degradation
stages. Further cell degradation resulted in recovery times much longer
than one night, appearing as irreversible degradation under real operational
conditions. At later degradation stages, very different dynamics were
observed: short-circuit current density and fill factor exhibited
a pronounced drop upon light turn-off but strong improvement under
subsequent illumination. The interplay of reversible and irreversible
degradation processes with different recovery dynamics was demonstrated
to result in changes in the cell’s diurnal PCE dependence during
its operational lifespan under real sunlight conditions.
We propose a new approach for assessing the lifetimes of perovskite photovoltaics based on daily energy output which accounts for reversible diurnal changes.
This study reports on how the degree of polymer order within a polymer/fullerene blend can be investigated by spectroscopic methods. Non-annealed blend compositions with 0-80 wt % fullerene content were analyzed using temperature dependent photoluminescence (PL) and room temperature spectroscopic ellipsometry (SE) measurements. To evaluate the SE data with respect to the optical order, an optical model was developed, including a lower and higher ordered polymer phase within a fullerene matrix. This was done using an effective medium approach describing the polymer by combining lower and higher ordered polymer properties (polymer-EMA). The polymer/fullerene blend was then evaluated using another EMA consisting of the polymer-EMA and the dielectric function of the disordered fullerene. The degree of optical order obtained by SE, was confirmed using another independent measurement, photoluminescence spectroscopy, according to the method of Francis C. Spano (2005). The volume fraction of the ordered polymer within the polymer-EMA was found to be between 70 and 60 vol % for fullerene contents lower than 20 wt % in the polymer/fullerene blend. Above 20 wt % fullerene, the optical order of the polymer strongly decreases all the way down to 0 vol %. In contrast to the complementary performed X-ray diffraction measurements, which address only the longrange structural order of the blends, we give quantitative information on the optical order, including information on the composition, that is, volume fractions of the higher and lower ordered polymer. The gained information on the tilt of the polymer molecules with respect to the substrate is discussed comparing XRD results from the literature with those obtained by our SE model. Finally, the developed model is used to describe the influence of the P3HT molecular weight on the optical order. Results obtained with our model were compared to the structural data and mobility data in the literature.
The influence of the polymer/fullerene blend ratio on the morphological properties of organic solar cells is investigated. Spectroscopic ellipsometry is applied as a tool for analyzing fullerene domains and the influence on polymer crystallinity within the film. Furthermore, the measurements are correlated with a percolation limit of as‐cast films at around 40 wt% fullerene content.
Recent efficiency
records of organic photovoltaics (OPV) highlight stability as a limiting
weakness. Incorporation of stabilizers is a desirable approach for
inhibiting degradationit is inexpensive and readily up-scalable.
However, to date, such additives have had limited success. We show
that β-carotene (BC), an inexpensive and green, naturally occurring
antioxidant, dramatically improves OPV stability. When compared to
nonstabilized reference devices, the accumulated power generation
of PTB7:[70]PCBM devices in the presence of BC increases by an
impressive factor of 6, due to stabilization of both the burn-in and
the lifetime, and by a factor of 21 for P3HT:[60]PCBM devices, owing
to a longer lifetime. Using electron spin resonance and time-resolved
near-IR emission spectroscopies, we probed radical and singlet oxygen
concentrations. We demonstrate that singlet oxygen sensitized by [70]PCBM
causes the “burn-in” of PTB7:[70]PCBM devices and that
BC effectively mitigates it. Our results provide an effective solution
to the problem that currently limits widespread use of OPV.
Crystalline molybdenum oxide layers as efficient and stable hole contacts in organic photovoltaic devices Ahmadpour, Mehrad; Cauduro, A. L. F.; Méthivier, C.; Kunert, B. ; Labanti, C.; Resel, R.; Engmann
We report on the improvement of long-term stability of organic solar cells (OPV) using hindered phenol based antioxidants as stabilizing additives. A set of seven commercially available hindered phenols are investigated for use in bulk-heterojunction OPV. Polymer:fullerene films based on poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) are characterized with respect to the initial power conversion efficiency and the long-term stability improvement under illumination in ambient conditions. FTIR spectroscopy is used to trace chemical degradation over time. OPV performance is recorded under ISOS-3 conditions, and an improved long-term performance of OPV devices, manifested in increased accumulated power generation (APG), is found for octadecyl 3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate. Using this additive, APG is increased by a factor of 3 compared to the reference. Observed differences in the stabilization of tested additives are discussed in terms of energetic trap states formation within the HOMO/LUMO gap of the photoactive material, morphological changes, and chemical structure.
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